DESCRIPTION (Adapted from the Applicant's Description): The long-range goal is to learn if CG has important physiological functions in addition to its role in "rescuing" the corpus luteum of human pregnancy. To eliminate CG without interfering with the actions of LH, the investigator is requesting funds to develop high titer antisera that can be used to passively block the activity of monkey CG (mCG) in pregnant monkeys. The goal of this study is to demonstrate that the investigator can prepare antisera to mCG in monkeys with little or no cross-reactivity with mLH. Aim 1 is to prepare two chimeric immunogens that are expected to be highly antigenic in mice and monkeys. Aim 2 involves testing the immunogenicity of these antigens in mice and then in sexually immature male cynomolgus monkeys. These monkeys will also provide a supply of antisera for future passive immunization of female monkeys.